<!DOCTYPE html>
<html lang="en">

    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Document</title>
    </head>

    <body>
        <input type="text"> 字符串长度: <span id="length">0</span>
        <script>
            //TODO 是编写代码，实时显示输入框中字符串的长度
        </script>
    </body>

</html>